1. Field of the Invention
The present invention relates to a coordinate input apparatus. More specifically, the present invention relates to a coordinate input apparatus to be used to control a connected computer or write characters, figures, and the like by pointing positions on an input surface by using a pointing tool or finger. Furthermore, the present invention relates to a technique of improving the performance of calculating pointed positions, or a technical field for faithfully reproducing, as handwriting or the like, the intension of an operator having performed a pointing operation.
2. Description of the Related Art
There is a coordinate input apparatus to be used to control a connected computer or write characters, figures, and the like by inputting coordinates on a coordinate input surface by using a pointing tool (for example, a dedicated input pen or finger).
Conventionally, various types of coordinate input apparatuses of this kind have been proposed or manufactured as touch panels. These coordinate input apparatuses are extensively used because a terminal such as a personal computer can easily be operated on the screen without using any special equipment.
There are various coordinate input methods such as a method using a resistance film, a method using an ultrasonic wave or electromagnetic wave, and a method using light. An example is a coordinate input apparatus in which a light emitting portion is formed at the distal end of a pointing tool as a dedicated writing tool, a light source of the pointing tool emits light when a touch input operation is performed using the pointing tool, and a light receiving portion formed at each corner of a coordinate input effective region detects the light, thereby calculating the touch input position of the writing tool (Japanese Laid-Open No. 2005-78433).
There is another known method in which a retroreflecting sheet is formed outside a coordinate input region, light from an illumination unit for emitting light is retroreflected by the retroreflecting sheet, and a light receiving unit for receiving light detects the light amount distribution. A position in the coordinate input region is pointed by a light-shielding object such as a finger for shielding light, thereby detecting the direction (angle) in (at) which the light is shielded, and determining the light-shielding position, that is, the coordinates of the input position (U.S. Pat. No. 4,507,557).
In order to detect the direction (angle), there is a method of detecting the peak of a light-shielding portion where the light receiving unit receives light by performing waveform processing such as differentiation on a detection signal waveform, thereby detecting the angle the light-shielding portion makes with the light receiving unit (Japanese Patent Laid-Open No. 2001-105671). In addition, there is a method of detecting one end and the other end of a light-shielding portion by comparison with a specific level pattern, thereby detecting the center of their coordinates (Japanese Patent Laid-Open No. 2001-142642).
Also, there is a method that prestores received light distributions with and without light projection from the light projecting unit in the initial state, and calculates coordinates from the change amount and change ratio of the reflected light amount by using a received light distribution obtained during coordinate detection and the two prestored received light distributions (Japanese Patent Laid-Open No. 2004-185283).
Each of the disclosed patent documents described above relates to an optical method of detecting a pointed position by using light, and has a configuration including a light receiving optical system. The focus (focal point) of the light receiving optical system is adjusted at a predetermined distance, for example, at the shortest distance or farthest distance between an object and the light receiving optical system within the input range of a coordinate input apparatus (Japanese Patent Laid-Open No. 2001-84090).
Furthermore, a detectable light intensity range (dynamic range) is set in a light detecting element of the light receiving optical system. If this range is exceeded, no normal operation may be performed any longer because, for example, electric charge overflows. In the coordinate input apparatus of this kind, therefore, the detection light amount must be adjusted such that the light intensity falls within the dynamic range of the light receiving element regardless of the direction. On known example is a method of changing the light transmittance in accordance with the incident direction by using an optical filter, thereby adjusting the light intensity (Japanese Patent Laid-Open No. 2001-282445).
In the optical coordinate input apparatus of this kind, a distance range as a detection target of the light receiving optical system is wide, and it is necessary to determine a pointed position and determine whether a pointing tool has actually touched a coordinate input surface, even from an image sensing signal in an out-of-focus state. Even in an out-of-focus state, the pointed position can be calculated with a relatively high accuracy by, for example, obtaining the center of gravity of an image of the pointing tool. However, it is very difficult to determine, from an out-of-focus image sensing signal, whether the pointing tool has actually touched the coordinate input surface, and only rough determination is possible.
Assume that an operator inputs a character “” in this state. FIGS. 14A and 14B are views for explaining a phenomenon that occurs in this state. Referring to FIG. 14A, the solid lines indicate portions where the operator has touched the coordinate input surface with the pointing tool such as a finger, and the dotted lines indicate portions where the operator hasn't touched the coordinate input surface. FIG. 14B shows loci (handwriting) displayed on a display unit by this operation.
A case in which it is determined by mistake that the operator has touched the positions of points b, c, and d in FIG. 14A although he or she hasn't will be explained below. In this case, as shown in FIG. 14B, “a short upward sweep” as an originally unintended locus is generated between points b and c. Also, if it is determined by mistake that the operator hasn't touched the position of point e although he or she actually has, an originally intended locus disappears near the position of point e as shown in FIG. 14B. This touch determination error causes an operation not intended by the operator, and hence significantly deteriorates the operability.
On the other hand, the operability remarkably improves if it is possible to form an optical system having a large depth of field and capable of focusing within the entire range. Unfortunately, this optical system includes lenses and optical elements having complicated shapes, or additionally includes an auto-focusing mechanism, and is difficult to inexpensively manufacture.
Furthermore, the coordinate input apparatuses have various sizes due to different product specifications or a series of sizes as a product lineup. In other words, a distance range to be detected changes in accordance with the specifications of each product, and this makes it difficult to use common parts in light receiving optical systems, and makes inexpensive manufacture impossible.
Moreover, the settings of jigs and tools for adjusting the optical axis and focus point of an optical system must be changed in accordance with the size of a coordinate input apparatus, and this unavoidably increases the number of assembly steps. Also, when the size of the coordinate input apparatus increases, the sizes of the jigs and tools increase accordingly, and this poses the problem of an installation space. In addition, the adjustment of the jigs and tools requires a considerable precision, and as a consequence the productivity decreases.